The invention concerns a pressure testing apparatus, a method for pressure testing chips on a wafer and use of such apparatuses.
Pressure sensors are micromechanical devices, which are fabricated in a wafer, typically from 100 to 300 mm in diameter and from 200 to 500 μm in thickness. The wafer is mounted in a test fixture so that the performance of the each individual pressure sensors can be tested. FIG. 1 shows a wafer. The pressure sensors, referred to as chips in the following text, are tested while still in wafer form or after separation from each other (by dicing), using an automatic testing apparatus.
The testing apparatus typically comprises a chuck for the wafer or the test fixture, a card support for a probe card and a positioning device for positioning the wafer or the test fixture during testing. Most of such testing apparatuses function at normal air pressure. However, for pressure sensors, testing at normal air pressure is insufficient. Therefore pressure sensors should be tested at low pressure and/or at overpressure.
In pressure sensor testing, low pressure may be applied towards the lower surface of the wafer so that the low pressure is produced through the chuck. For proper function, the contact surfaces of the sensor should be located on the opposite side of the active pressure surface in the sensor.
Producing overpressure through the chuck is possible, but then the wafer should be held against the chuck using a force at least equal to the force of pressure below the wafer. Since wafers are typically very thin, breaking of the wafer already at very low overpressure is possible.
For overpressure testing, the active pressure surface and its contacts may be located on the same side of the wafer. In a known technique, a conduit in association with the probes of the probe card, for example needles, is brought near the surface of the sensor. Controlling exactly the flow of blowing, a specific area on the wafer, for example a pressure sensor, is pressurized. However, with flow control it is difficult to obtain an exact pressure. Furthermore, by using such known technique it is difficult to achieve a high pressure of over 10 bars.
Another known technique is to force a conduit closely against the surface of the wafer so that the conduit forms a pressure chamber together with the wafer surface. Thus, pressure control is easier. However, in most cases no high contact force is allowed to be directed toward the wafer surface.
In U.S. Pat. No. 4,777,716 a chuck is provided with a seal circumferentially around the wafer. The probe card is located above the chuck below the cover. A pressure sensor to be tested is placed in contact with the probes by moving the chuck upward against the structure above it. Thereby, the chuck becomes tightened against the upper structure and forms a hermetically sealed chamber together with the upper structure. The level of pressure can be varied with aid of an external pressure system. If testing is desired to be done with a great overpressure the pressure force is encountered due to large pressurized area which force tend to open the chamber during testing. The pressure force is directed towards the positioning device of the chuck. Thereby the positioning device should be made heavy by its construction. Another problem is that the force between the sensor and the probes of the probe card cannot be adjusted by the vertical movement of the chuck. Furthermore, the capacity for testing is reduced by the structure of the apparatus. For positioning from one sensor to another sensor, the pressure chamber of the apparatus has to be opened. After closing the chamber it has to be pressurized again.
U.S. Pat. No. 6,373,271 presents another typical solution wherein the above mentioned problems are decreased so that the part of the chuck, which is tightened against the upper structure, is flexible allowing the horizontal movements of the chuck within the specific limits. Thus, opening of a hermetically sealed pressurized chamber between the movements is not required. Flexible construction enables adjustment of applied force between the sensor and the probes of the probe card since the vertical movement is not totally fixed. However, a disadvantage is the great pressure force, which is applied towards the wafer chuck.
One known technique is shown in FIG. 3. The positioning device for the testing apparatus is arranged inside a cylindrical pressure chamber. The pressure chamber experiences a movement due to a change in an applied pressure in the chamber and this movement is conducted to the positioning device through the walls unless the thickness of the wall is well oversized. Thus, the positioning device cannot be fastened rigidly to the walls of the pressure chamber otherwise the movement may cause problems in its positioning accuracy.